The trend of industry automation has swept the globe in recent years, which significantly enhances the productivity and yield of products. Under the trend of automation, a steady, reliable and high precision sensing technique is becoming more and more important. The most common precision sensing technique in industrial automation is laser displacement measuring technique which uses a laser as a sensor to measure displacement. There are three basic types of laser displacement measuring methods: time-of-flight, phase difference and triangulation measuring method.
Time-of-flight measuring method uses a modulated laser light to measure the distance. Distance could be determined by the time delay between the same modulation signals between being emitted and observed. Phase difference measuring method measures the distance by the modulation signal phase difference between the emitted and received laser light. The triangulation measuring method, disclosed for example in U.S. Pat. No. 4,864,147 “Optically Scanning Displacement Sensor with Linearity Correction Means” and U.S. Pat. No. 4,897,536 “Optical Axis Displacement Sensor with Cylindrical Lens Means”, use laser as a index to measure the distance by the geometrical relation between the object and optical sensor as photodiodes(PD) array. However, triangulation measuring method has strict requirements for structural stability of the system and the sensibility of the PD array sensor. The applications of the above-described laser measuring methods are different, but they all demand highly sensible receivers and precision electronic circuits, causing the cost of the products becoming too high.
In addition, U.S. Pat. No. 4,828,390 “Optical Axis Displacement Sensor” proposes an optical axis displacement sensing technique, which takes the optical axis of the instrument structure for reference, i.e., takes the optical axial position of the receiver as zero. The position variation of the measured object can be deduced by obtaining the position variation of the receiver. This technique also requires high structural stability of the system and accurate arrangement of the lens. The technique further requires a moving component, which not only increases the cost, but also reduces structural stability of the system.
Furthermore, U.S. Pat. No. 5,424,834 “Optical Displacement Sensor for Measurement of Shape and Coarseness of a Target Workpiece Surface” proposes a laser confocal scanning technique. According to the optical confocal manner, different positions will have different distributions of the light intensity, indicating position variation of the measured object, thus the displacement of the measured object can be measured. However, problems such as strict structural stability of the system and component assembling (e.g., pinhole focusing) still exist in such technique, which is adverse to system stability and reduction of cost.
Accordingly, there exists a strong need in the art for a simple measuring method that obtains better displacement measuring result with reduced cost.